How to sort a HashMap in Java

问题:

How are we able to sort a HashMap<key, ArrayList>?

I want to sort on the basis of a value in the ArrayList.

回答1:

Do you have to use a HashMap? If you only need the Map Interface use a TreeMap

---

If you want to sort by comparing values in the HashMap. You have to write code to do this, if you want to do it once you can sort the values of your HashMap:

Map<String, Person> people = new HashMap<>();
Person jim = new Person(\"Jim\", 25);
Person scott = new Person(\"Scott\", 28);
Person anna = new Person(\"Anna\", 23);

people.put(jim.getName(), jim);
people.put(scott.getName(), scott);
people.put(anna.getName(), anna);

// not yet sorted
List<Person> peopleByAge = new ArrayList<>(people.values());

Collections.sort(peopleByAge, Comparator.comparing(Person::getAge));

for (Person p : peopleByAge) {
    System.out.println(p.getName() + \"\\t\" + p.getAge());
}

If you want to access this sorted list often, then you could insert your elements into a HashMap<TreeSet<Person>>, though the semantics of sets and lists are a bit different.

回答2:

Sorted List by hasmap keys:

SortedSet<String> keys = new TreeSet<String>(myHashMap.keySet());

Sorted List by hashmap values:

SortedSet<String> values = new TreeSet<String>(myHashMap.values());

In case of duplicated map values:

List<String> mapValues = new ArrayList<String>(myHashMap.values());
Collections.sort(mapValues);

Good Luck!

回答3:

http://snipplr.com/view/2789/sorting-map-keys-by-comparing-its-values/

get the keys

List keys = new ArrayList(yourMap.keySet());

Sort them

 Collections.sort(keys)

print them.

In any case, you can\'t have sorted values in HashMap (according to API This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time ].

Though you can push all these values to LinkedHashMap, for later use as well.

回答4:

Seems like you might want a treemap.

http://docs.oracle.com/javase/7/docs/api/java/util/TreeMap.html

You can pass in a custom comparator to it if that applies.

回答5:

In Java 8:

Comparator<Entry<String, Item>> valueComparator = 
    (e1, e2) -> e1.getValue().getField().compareTo(e2.getValue().getField());

Map<String, Item> sortedMap = 
    unsortedMap.entrySet().stream().
    sorted(valueComparator).
    collect(Collectors.toMap(Entry::getKey, Entry::getValue,
                             (e1, e2) -> e1, LinkedHashMap::new));

Using Guava:

Map<String, Item> map = ...;
Function<Item, Integer> getField = new Function<Item, Integer>() {
    public Integer apply(Item item) {
        return item.getField(); // the field to sort on
    }
};
comparatorFunction = Functions.compose(getField, Functions.forMap(map));
comparator = Ordering.natural().onResultOf(comparatorFunction);
Map<String, Item> sortedMap = ImmutableSortedMap.copyOf(map, comparator);

回答6:

Custom compare function which includes functionality for the Turkish alphabet or other different languages than english.

public <K extends Comparable,V extends Comparable> LinkedHashMap<K,V> sortByKeys(LinkedHashMap<K,V> map){
    List<K> keys = new LinkedList<K>(map.keySet());
    Collections.sort(keys, (Comparator<? super K>) new Comparator<String>() {
        @Override
        public int compare(String first, String second) {
            Collator collator = Collator.getInstance(Locale.getDefault());
            //Collator collator = Collator.getInstance(new Locale(\"tr\", \"TR\"));
            return collator.compare(first, second);
        }
    });

    LinkedHashMap<K,V> sortedMap = new LinkedHashMap<K,V>();
    for(K key: keys){
        sortedMap.put(key, map.get(key));
    }

    return sortedMap;
}

here is the using example as the following

LinkedHashMap<String, Boolean> ligList = new LinkedHashMap<String, Boolean>();
ligList = sortByKeys(ligList);

回答7:

Without any more information, it\'s hard to know exactly what you want. However, when choosing what data structure to use, you need to take into account what you need it for. Hashmaps are not designed for sorting - they are designed for easy retrieval. So in your case, you\'d probably have to extract each element from the hashmap, and put them into a data structure more conducive to sorting, such as a heap or a set, and then sort them there.

回答8:

If you want to combine a Map for efficient retrieval with a SortedMap, you may use the ConcurrentSkipListMap.

Of course, you need the key to be the value used for sorting.

回答9:

have you considered using a LinkedHashMap<>()..?

  public static void main(String[] args) {
    Map<Object, Object> handler = new LinkedHashMap<Object, Object>();
    handler.put(\"item\", \"Value\");
    handler.put(2, \"Movies\");
    handler.put(\"isAlive\", true);

    for (Map.Entry<Object, Object> entrY : handler.entrySet())
        System.out.println(entrY.getKey() + \">>\" + entrY.getValue());

    List<Map.Entry<String, Integer>> entries = new ArrayList<Map.Entry<String, Integer>>();
    Collections.sort(entries, new Comparator<Map.Entry<String, Integer>>() {
        public int compare(Map.Entry<String, Integer> a,
                Map.Entry<String, Integer> b) {
            return a.getValue().compareTo(b.getValue());
        }
    });
}

results into an organized linked object.

 item>>Value
 2>>Movies
 isAlive>>true

check the sorting part picked from here..

回答10:

Sorting HashMap by Value:

As others have pointed out. HashMaps are for easy lookups if you change that or try to sort inside the map itself you will no longer have O(1) lookup.

The code for your sorting is as follows:

class Obj implements Comparable<Obj>{
    String key;
    ArrayList<Integer> val;
    Obj(String key, ArrayList<Integer> val)
    {
    this.key=key;
    this.val=val;
    }
    public int compareTo(Obj o)
    {
     /* Write your sorting logic here. 
     this.val compared to o.val*/
     return 0;
    }
}

public void sortByValue(Map<String, ArrayList<>> mp){

    ArrayList<Obj> arr=new ArrayList<Obj>();
    for(String z:mp.keySet())//Make an object and store your map into the arrayList
    {

        Obj o=new Obj(z,mp.get(z));
        arr.add(o);
    }
    System.out.println(arr);//Unsorted
    Collections.sort(arr);// This sorts based on the conditions you coded in the compareTo function.
    System.out.println(arr);//Sorted
}

回答11:

I have developed a class which can be used to sort a map on the basis of keys and values. The basic idea is if you have sort a map using keys then create a TreepMap from your Map which will sort the map by keys. And in case of sorting by values create a list from entrySet and sort the list using comparator interface.

Here is the full solution :

public static void main(String[] args) {
    Map<String, Integer> unSortedMap = new LinkedHashMap<String, Integer>();
    unSortedMap.put(\"A\", 2);
    unSortedMap.put(\"V\", 1);
    unSortedMap.put(\"G\", 5);
    System.out.println(\"Unsorted Map :\\n\");
    for (Map.Entry<String, Integer> entry : unSortedMap.entrySet()) {
        System.out.println(entry.getKey() + \"   \" + entry.getValue());
    }
    System.out.println(\"\\n\");
    System.out.println(\"Sorting Map Based on Keys :\\n\");
    Map<String, Integer> keySortedMap = new TreeMap<String, Integer>(unSortedMap);
    for (Map.Entry<String, Integer> entry : keySortedMap.entrySet()) {
        System.out.println(entry.getKey() + \"   \" + entry.getValue());
    }
    System.out.println(\"\\n\");
    System.out.println(\"Sorting Map Based on Values :\\n\");
    List<Entry<String, Integer>> entryList = new ArrayList<Entry<String, Integer>>(unSortedMap.entrySet());
    Collections.sort(entryList, new Comparator<Entry<String, Integer>>() {

        @Override
        public int compare(Entry<String, Integer> obj1, Entry<String, Integer> obj2) {
            return obj1.getValue().compareTo(obj2.getValue());
        }
    });
    unSortedMap.clear();
    for (Entry<String, Integer> entry : entryList) {
        unSortedMap.put(entry.getKey(), entry.getValue());
        System.out.println(entry.getKey() + \"   \" + entry.getValue());
    }
}

Code is properly tested :D

回答12:

A proper answer.

HashMap<Integer, Object> map = new HashMap<Integer, Object>();

ArrayList<Integer> sortedKeys = new ArrayList<Integer>(map.keySet());
Collections.sort(sortedKeys, new Comparator<Integer>() {
  @Override
  public int compare(Integer a, Integer b) {
    return a.compareTo(b);
  }
});

for (Integer key: sortedKeys) {
  //map.get(key);
}

Note that HashMap itself cannot maintain sorting, as other answers have pointed out. It\'s a hash map, and hashed values are unsorted. You can thus either sort the keys when you need to and then access the values in order, as I demonstrated above, or you can find a different collection to store your data, like an ArrayList of Pairs/Tuples, such as the Pair found in Apache Commons:

https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/tuple/Pair.html

回答13:

Sorting by key:

public static void main(String[] args) {
    Map<String,String> map = new HashMap<>();

    map.put(\"b\", \"dd\");
    map.put(\"c\", \"cc\");
    map.put(\"a\", \"aa\");

    map = new TreeMap<>(map);

    for (String key : map.keySet()) {
        System.out.println(key+\"=\"+map.get(key));
    }
}

回答14:

I developed a fully tested working solution. Hope it helps

import java.io.BufferedReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.StringTokenizer;


public class Main {
    public static void main(String[] args) {
    try {
        BufferedReader in = new BufferedReader(new java.io.InputStreamReader           (System.in));
            String str;

        HashMap<Integer, Business> hm = new HashMap<Integer, Business>();
        Main m = new Main();


        while ((str = in.readLine()) != null) {


            StringTokenizer st = new StringTokenizer(str);
            int id = Integer.parseInt(st.nextToken());    // first integer
            int rating = Integer.parseInt(st.nextToken());    // second 

            Business a = m.new Business(id, rating);


            hm.put(id, a);


            List<Business> ranking = new ArrayList<Business>(hm.values());

            Collections.sort(ranking, new Comparator<Business>() {

                public int compare(Business i1, Business i2) {
                    return i2.getRating() - i1.getRating();
                }
            });

            for (int k=0;k<ranking.size();k++) {
                System.out.println((ranking.get(k).getId() + \" \" + (ranking.get(k)).getRating()));
            }


        }
        in.close();

    } catch (IOException e) {
        e.printStackTrace();
    }


}
public class Business{

    Integer id;
    Integer rating;

    public Business(int id2, int rating2)
    {
        id=id2;
        rating=rating2;

    }

    public Integer getId()
    {
        return id;
    }
    public Integer getRating()
    {
        return rating;
    }


}
}

回答15:

HashMap doesnt maintain any order, so if you want any kind of ordering, you need to store that in something else, which is a map and can have some kind of ordering, like LinkedHashMap

below is a simple program, by which you can sort by key, value, ascending ,descending ..( if you modify the compactor, you can use any kind of ordering, on keys and values)

package com.edge.collection.map;

import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

public class SortMapByKeyValue {
Map<String, Integer> map = new HashMap<String, Integer>();

public static void main(String[] args) {

    SortMapByKeyValue smkv = new SortMapByKeyValue();
    smkv.createMap();

    System.out.println(\"After sorting by key ascending order......\");
    smkv.sortByKey(true);

    System.out.println(\"After sorting by key descindeng order......\");
    smkv.sortByKey(false);

    System.out.println(\"After sorting by value ascending order......\");
    smkv.sortByValue(true);

    System.out.println(\"After sorting by value  descindeng order......\");
    smkv.sortByValue(false);

}

void createMap() {
    map.put(\"B\", 55);
    map.put(\"A\", 80);
    map.put(\"D\", 20);
    map.put(\"C\", 70);
    map.put(\"AC\", 70);
    map.put(\"BC\", 70);
    System.out.println(\"Before sorting......\");
    printMap(map);
}

void sortByValue(boolean order) {

    List<Entry<String, Integer>> list = new LinkedList<Entry<String, Integer>>(map.entrySet());
    Collections.sort(list, new Comparator<Entry<String, Integer>>() {
        public int compare(Entry<String, Integer> o1, Entry<String, Integer> o2) {
            if (order) {
                return o1.getValue().compareTo(o2.getValue());
            } else {
                return o2.getValue().compareTo(o1.getValue());

            }
        }
    });
    Map<String, Integer> sortedMap = new LinkedHashMap<String, Integer>();
    for (Entry<String, Integer> entry : list) {
        sortedMap.put(entry.getKey(), entry.getValue());
    }
    printMap(sortedMap);

}

void sortByKey(boolean order) {

    List<Entry<String, Integer>> list = new LinkedList<Entry<String, Integer>>(map.entrySet());
    Collections.sort(list, new Comparator<Entry<String, Integer>>() {
        public int compare(Entry<String, Integer> o1, Entry<String, Integer> o2) {
            if (order) {
                return o1.getKey().compareTo(o2.getKey());
            } else {
                return o2.getKey().compareTo(o1.getKey());

            }
        }
    });
    Map<String, Integer> sortedMap = new LinkedHashMap<String, Integer>();
    for (Entry<String, Integer> entry : list) {
        sortedMap.put(entry.getKey(), entry.getValue());
    }
    printMap(sortedMap);
}

public void printMap(Map<String, Integer> map) {
    // System.out.println(map);
    for (Entry<String, Integer> entry : map.entrySet()) {
        System.out.println(entry.getKey() + \" : \" + entry.getValue());
    }
}
}

here is the git link

回答16:

Convert hashmap to a ArrayList with a pair class

Hashmap<Object,Object> items = new HashMap<>();

to

List<Pair<Object,Object>> items = new ArrayList<>();

so you can sort it as you want, or list sorted by adding order.

回答17:

Sorting HashMap by value in Java:

public class HashMapSortByValue {
    public static void main(String[] args) {

        HashMap<Long,String> unsortMap = new HashMap<Long,String>();
            unsortMap.put(5l,\"B\");
            unsortMap.put(8l,\"A\");
            unsortMap.put(2l, \"D\");
            unsortMap.put(7l,\"C\" );

            System.out.println(\"Before sorting......\");
            System.out.println(unsortMap);

            HashMap<Long,String> sortedMapAsc = sortByComparator(unsortMap);
            System.out.println(\"After sorting......\");
            System.out.println(sortedMapAsc);

    }

    public static HashMap<Long,String> sortByComparator(
            HashMap<Long,String> unsortMap) {

            List<Map.Entry<Long,String>> list = new LinkedList<Map.Entry<Long,String>>(
                unsortMap.entrySet());

            Collections.sort(list, new Comparator<Map.Entry<Long,String>> () {
                public int compare(Map.Entry<Long,String> o1, Map.Entry<Long,String> o2) {
                    return o1.getValue().compareTo(o2.getValue());
                }
            });

            HashMap<Long,String> sortedMap = new LinkedHashMap<Long,String>();
            for (Entry<Long,String> entry : list) {
              sortedMap.put(entry.getKey(), entry.getValue());
            }
            return sortedMap;
          }

}